You expressly said MOUNTING. Having external rads isn't ugly, you can put them in hidden places where they don't bother you and may look nice. However, mounting a rad externally never really looks good to me. Unless you have a full shroud or something. Seeing the tubes, wires and naked fans just seems like unfinished business to me.

I think it depends on the setup...I have my rad mounted externally because it fits my case well. I have a Corsair 600t White with an external rad, but I used white tubing. So, since the case is black on white and the rad is black on white, I think it looks really nice and adds to the case, but beauty is in the eyes of the beholder.